The subject matter of this application generally relates to systems and methods that improve the performance of multiple amplifiers contained within a node of a CATV network.
Cable television (CATV) delivery systems provide a variety of media content, such as video, data, voice, or high-speed Internet services to subscribers. The CATV provider typically delivers the media content from a head end to its subscriber's client devices over a transmission network such as a coaxial network, a fiber optic network, or a hybrid fiber/coax (HFC) network. Requirements for data throughput (or bandwidth) in these CATV networks are growing exponentially as customers demand more content, data services, etc. Though improvements in encoding efficiencies and transport protocols have thus far allowed cable operators to keep pace with subscriber and competitive demands, it is important to continue the analysis of the various network elements that can enhance or inhibit the overall performance of CATV systems.
Generally (RF) amplifiers within the cable television network operate in what is referred to as a “class A” mode of operation, which provides a very high fidelity signal, often quantified in terms of signal-to-noise and signal-to-2nd, 3rd, 4th, 5th . . . harmonic distortion products. In a class A amplifier, the transistor is always biased “ON” so that it conducts during one complete cycle of the input signal waveform, producing minimum distortion and maximum amplitude to the output since there is no crossover distortion to the output waveform, even during the negative half of the cycle. However, because the transistor is always biased “ON,” one of the main disadvantages of class A amplifiers is that their efficiency is very low as the constant and usually large bias currents cause a considerable amount of power to be lost. It is desirable to reduce power dissipation by minimizing the bias current of amplifiers be it class A amplifiers or more complex class AB or class B amplifiers but this does degrade the amplifier distortion performance such that some form of distortion cancellation is generally needed.
To avoid this excessive power loss, class B amplifiers may be used, which have a pair of transistors that each conduct alternatingly only for one half cycle of the input signal. Since the active devices are switched off for half the input cycle, the active device dissipates less power and hence the power efficiency is improved. However, since each active device truncates half the input signal and the output signal is the sum of the outputs of the active devices, such that the half waves are constructively added to full waves, cross-over distortion that occurs when one device switches off and the other switches on is high in class B amplifiers.
Class AB amplifiers similarly tend to produce distortion. In class AB amplifiers for RF applications, typically an output transformer is used where a pair of transistors are operated in a push-pull configuration and each of the transistors is driven with a signal set relative to ground, as opposed to some floating node. Each transistor alternatingly conducts a little more than one half cycle of the input signal such that for small signals there is an overlap region where both transistors are active, as in a class A amplifier with low bias. For large signals the operation is as with a class B amplifier. The small bias current for class A operation with small signals does increase power dissipation somewhat compared to a pure class B amplifier but it can still be much less than for a class A amplifier. A comparison between the input and a fraction of the output signals is used to generate a feedback signal that is theoretically used to reduce distortion. But in implementation this is often difficult. Feedback in an RF amplifier is generally limited, due to the high signal bandwidth that needs to be amplified. Too much delay in the feedback path combined with a high gain for the correction signal, as would be needed for good distortion suppression, will lead to amplifier oscillation, which produces distortions due to the transitions in the operating mode of the output transistors. Secondly, turning an RF transistor off can produce very high distortions when the transistor needs to be turned on again. This makes it difficult to use a class AB amplifier in a CATV application that requires low distortion and high RF bandwidth.
What is desired, therefore, are improved systems and methods for reducing distortion produced by amplifiers in CATV systems.